This application relates generally to optical communications and more specifically to techniques for manufacturing and packaging multi-wavelength distributed feedback (DFB) semiconductor laser (laser diode) arrays. All patent documents and other publications referred to herein are incorporated by reference in their entirety for all purposes.
Everywhere around the world, the ways people connectxe2x80x94through voice, video, and dataxe2x80x94are radically changing through rapid advances of communication (telephony and computing) technologies. These technologies may vary widely in applications, yet every technology shares a common need: an ever-increasing need for more and more speed and bandwidth from 10 Mbit/sec to 100 Gbit/sec and beyond. The need for increasing bandwidth is equally compelling both in wireless and fiber-optic transmission networks.
While wireless technologies deliver freedom to communicate without any wire, they may be limited to only low to moderate bandwidth applications at the present time. For high bandwidth applications (beyond 10 Gbit/s), wired fiber-optic technology appears to be the only cost-effective solution at this time. For over a century standard copper cable has been used for telecommunication, but fiber-optic (cylindrical conduits of glass) can transmit voice, video, and data 100 times faster than standard copper cable. Unfortunately, only a minute fraction of the capacity of fiber-optic technology has been realized as of today due to limitation of optical-to-electronic and vice versa conversion methods.
With the invention of the erbium-doped optical fiber amplifier, the need for optical-to-electronic conversion in the networks is minimized. Thus by maintaining signal in the optical format and utilizing a wavelength division multiplexed/demultiplexed technology (WDM/WDDM, or sometimes simply WDM)xe2x80x94multiple different wavelengths (moderate bit rate on separate and distinct wavelengths) over the same optical-fiber, a large aggregate bit rate can be achieved.
Allowing a uniform amplification across many wavelengths, it is possible to transmit more than 40 wavelength (assuming a 100 GHz or 0.8 nm wavelength separation with each wavelength operating at a bit rate of 2.5 Gbit/s to 10 Gbit/s). WDM systems that are being manufactured today utilize discrete wavelength-specific components (transmitters/multiplexers and filters/demultiplexers).
Current wavelength normalized and average WDM system price per wavelength is on the order of $60,000 for ultra long-distance (approx 600 km) telecommunication applications and on the order of $25,000 for short-distance (approx 60 km) telecommunications applications. As the price of WDM components drops and the cost of deploying WDM technology becomes economical, it becomes possible to deploy WDM technology in the metropolitan, local telephone, fiber-to-the-home, and data communication markets.
Linear and curved gratings are the key elements of many advanced active and passive opto-electronic devices, such as distributed feedback (DFB) lasers, distributed Bragg reflector (DBR) lasers, unstable resonator lasers with curved gratings, vertically focused lasers, and filters. These advanced devices play significant roles in the fiber-optic communication systems for telephony, and computing.
There are number of known techniques for fabricating gratings of the type required, but they are typically characterized by a number of disadvantages. For example, direct write electron beam lithography has the advantages of fine pitch control and the ability to produce quarter-wavelength or finer phase shifts and arbitrary shaped gratings. However, it is characterized by high equipment expense and low throughput, and subjects the wafers to potential material damage due to the impingement of the energetic electron beam. Other approaches using a binary phase mask have the advantages of high throughput, fine pitch control, and the ability to produce quarter-wavelength or finer phase shifts and arbitrary shaped gratings. However, they can be characterized by complex fabrication procedures, and are limited to grating pitches commensurate with the mask pitch (say 200 nm).
The present invention provides a robust process to manufacture phase masks which can be used to make both linear and curved gratings of single or multiple submicron pitches (including continuously varying pitches), with or without any abrupt quarter-wavelength shifts (or gradually varying finer phase shifts) simultaneously on the wafer/substrate. This allows practical commercial fabrication of multi-wavelength laser diode arrays (laser chips). The invention also provides techniques for fabricating durable and reliable laser chips, efficiently packaging them and interfacing them to laser driver chips. The laser chips can be made using standard semiconductor processes, although embodiments of the invention further enhance some of such process to provide improved manufacturability and laser chip reliability.
The present invention utilizes direct write electron or ion-beam lithography of two times the required submicron pitches of linear and curved gratings (with or without phase-shifted regions) on commercially available xcfx80 phase-shifting material on a quartz substrate and wet or dry etching of the xcfx80 phase-shifting material. Wet or dry etching of the xcfx80 phase-shifting material produces an exact xcfx80 phase shift which is necessary to produce a zero order nulled xcfx80 phase-shifted phase mask. In an alternative embodiment, a xcfx80 phase-shift mask is produced by direct writing on a quartz substrate and etching the quartz substrate to a very precise depth to cancel the zero order beams (transmitted and diffracted). The invention thus relaxes critical pitch dimensions for electron or ion-beam lithography fabrication of less than 200 nm pitch linear and/or curved gratings.
The present invention also provides an improved ridge laser structure having metal shoulders on either side of the laser""s active region. The shoulders are formed over an insulating layer, but one of the shoulders is electrically connected by contact metal to the ridge waveguide semiconductor material.
The present invention also provides an improved technique for coupling the information-bearing signal to the laser chip with very high fidelity. This is achieved by designing a circuit that can carry multiple signals at very high frequencies (say 10 GHz) without interference (known as crosstalk) degradation. According to this aspect of the invention, metallized via holes connect metal structures above a substrate to a backside ground plane below the substrate. In one embodiment, the metal structures are ground lines interspersed with RF/DC transmission lines on the top surface of the substrate. The ground lines are perforated by the metallized vias. In another embodiment, the vias can be disposed in pairs distributed along the RF/DC transmission line, with one via in each pair on one side of the RF/DC transmission line and the other via in the pair on the other side. The metal structures in this case can be individual wire arches overlying the RF/DC transmission line and extending into the vias on either side.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.